Platinum (Pt) has become an attractive material for use in integrated circuits because of its desirable chemical and mechanical properties, having a very low reactivity and being inert to oxidation. Platinum also has a low leakage current and a high electrical conductivity. Further, platinum is known to have a notably high work function. The work function is an important feature of a DRAM capacitor electrode material and, when quantified, it denotes the energy required to remove one electron from the metal. Advanced DRAM capacitors are characterized by a dominant leakage mechanism, known as the Schottky emission from metal into the dielectric, so that metals, like platinum, with high work function produce less leakage.
Deposition of a metal layer generally occurs through one of the following techniques: chemical vapor deposition (CVD); physical vapor deposition (PVD), also known as sputtering; or electrochemical deposition. CVD involves high temperatures which can lead to cold creep effects and an increased chance of impurity contamination over other methods, and sputtering has problems yielding sufficient step coverage and density at small line widths. Electrochemical deposition, however, offers a more controlled environment to reduce the chance of contamination, and a process that takes place with minor temperature fluctuations. Electrochemical deposition provides more thorough coverage, fewer physical flaws, and reduces separation between the layers.
There are several known electrochemical deposition processes used to form platinum interconnects and/or capacitor structures, for example capacitor electrodes. Electroplating of platinum onto a substrate is now a common practice in the manufacture of various platinum interconnect and/or capacitor electrodes. Such an electroless plating bath typically includes (1) water; (2) a soluble compound containing platinum to be deposited onto the substrate of interest; (3) a complexing agent for the corresponding platinum ions, which prevents chemical reduction of the platinum ions in solution while permitting selective chemical reduction on a surface of the substrate; (4) a chemical reducing agent for the platinum ions; (5) a buffer for controlling the pH; and (6) small amounts of additives, such as surfactants or stabilizers.
A disadvantage of the platinum plating bath described above is that conformal plating of a platinum electrode of a container capacitor, for example, requires low current densities for platinum plating. However, at low current densities, platinum Pt+4 ions get converted into Pt+2 ions which do not plate out. As a result, the converted Pt+2 ions remain in the plating solution and dissociate into platinum when current is passed thorough the solution. To remedy this drawback, plating at higher current densities has been proposed, but this deposition is not suitable for capacitor applications, such as electrode formation.
There is needed, therefore, a simple and inexpensive method of operating a plating bath at low current densities and without degrading the plating bath.